Catalysts for the production of hydrocarbons from carbon monoxide and water

ABSTRACT

A method of converting low H 2  /CO ratio syngas to carbonaceous products comprising reacting the syngas with water or steam at 200° to 350° C. in the presence of a metal catalyst supported on zinc oxide. Hydrocarbons are produced with a catalyst selected from cobalt, nickel or ruthenium and alcohols are produced with a catalyst selected from palladium, platinium, ruthenium or copper on the zinc oxide support. The ratio of the reactants are such that for alcohols and saturated hydrocarbons: 
     
         (2n+1)≧x≧O 
    
     and for olefinic hydrocarbons: 
     
         2n≧x≧O 
    
     where n is the number of carbon atoms in the product and x is the molar amount of water in the reaction mixture.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention pursuant toContract No. DE-AC02-76CH00016 between the U.S. Department of Energy andAssociated Universities, Inc.

BACKGROUND OF THE INVENTION

Syngas, a mixture of carbon monoxide and hydrogen, is widely employed inthe catalytic production of hydrocarbons, alcohols and mixtures of theseproducts.

Typically for the production of saturated hydrocarbons and olefins thereaction is carried out in a reaction inert liquid medium containingsuspended catalyst. Typical catalysts include iron and cobalt normallyabsorbed on a carrier such as thorium, magnesium oxide or kieselguhr. Toproduce alcohols, noble metals on the same supports are often used.

The usual source of syngas is the gasification reaction in which watervapor is reacted with a source of carbon, usually coal, under knownconditions. The reaction is endothermic and a common source of heat forthe reaction is to burn some of the coal by reaction with oxygen. Thetheoretical reaction including the burning is:

    2C+H.sub.2 O+1/2O.sub.2 ♯2CO+H.sub.2

so that the theoretical ratio of carbon monoxide to hydrogen in thereaction product is 2:1.

For the production of methane, the theoretical reaction is:

    CO+3H.sub.2 ♯H--CH.sub.2 --H+H.sub.2 O

and the general reaction for the production of higher saturatedhydrocarbons is:

    nCO+(2n+1)H.sub.2 ♯H--CH.sub.2 --.sub.n H+nH.sub.2 O

It is thus apparent that the ideal ratio of hydrogen to carbon monoxidein feed gas for the production of saturated hydrocarbons is between 2:1and 3:1. The theoretical ratio in syngas, as shown above is only 1:2. Infact the "real world" actual ratio in the usual industrial operation isabout 0.5:1 to 0.8:1 in syngas.

To compensate for the missing hydrogen, the usual commercial practice isto mix the syngas with water vapor as an additional hydrogen source.

The theoretical reaction which takes place in this case is representedby the equation: ##STR1## where n is the number of carbon atoms in thesaturated hydrocarbon produced, and x is the number of moles of water.

To conduct this reaction at maximum efficiency using the low ratiosyngas industrially available, iron and cobalt are the most widelyemployed catalysts. Iron is advantageous because it is "flexible" whichmeans that it will accept and operate reasonably efficiently with lowratio syngas in which the ratio of hydrogen to carbon monoxide variesover a wide range. However, iron requires the use of high temperaturesand the rate of reaction is relatively low. The use of cobalt gives riseto higher reaction rates, but cobalt is not flexible and is sensitive totemperatures above 250° C.

The generalized equation for the production of olefinic hydrocarbons is:##STR2##

The problems experienced with this reaction are similar to the problemsexperienced for the production of saturated hydrocarbons.

When a syngas-water mixture is employed to produce alkanols, thetheoretical equation is: ##STR3##

The catalysts normally employed in the reaction have been palladium,platinum, iridium, copper and ruthenium with or without a carrier. Thesecatalysts have proved to be flexible like iron, but they also suffer thesame disadvantages as iron. They require high temperatures, and the rateof reaction is low.

DESCRIPTION OF THE INVENTION

It has now been discovered that the reactions discussed above can beconducted with low ratio syngas, at low temperature, at good rates togive high yields of the desired products by the use of selectedcatalysts under defined conditions. The process of the invention is alsovery flexible, that is it is able to accommodate itself to a wide rangeof hydrogen to carbon monoxide ratios.

More specifically, it has been discovered that saturated hydrocarbonscan be produced in good yield if the reaction between carbon monoxide,hydrogen and water is suspended preferably at a temperature of from 225°to 275° C. under conditions such that the relative concentration of thereactants is expressed by the formula:

    (2n+1)≧x≧0

where n and x have the same meaning as above.

For the production of olefinic hydrocarbons the operativemonounsaturated formula is:

    2n≧x≧0

For the production of alkanols the operative formula is the same as forthe production of saturated hydrocarbons.

The temperature range for the production of olefins and alkanols is thesame as for the production of saturated hydrocarbons.

The catalysts employed on this invention are all supported on zincoxide. For the production of saturated hydrocarbons and olefins, thecatalysts are cobalt, nickel, ruthenium, or mixtures thereof supportedon zinc oxide. For the production of alkanols, the catalysts on the zincoxide support are palladium, platinum, iridium, copper, rhodium, ormixtures thereof.

The supported catalysts are prepared by the deposition of the catalyticmetal on zinc oxide. Typically, this is accompanied by impregnation ofthe support with an aqueous solution of a salt of the particular metal.Such preparations are shown, for example, in U.S. Pat. No. 3,988,334,incorporated herein by reference. In the usual process, as applied tothe preparation of a supported catalyst of this invention, zinc oxide inparticulate form is immersed in an aqueous solution of a salt of theselected metal. After sufficient time for impregnation of the support,the mixture is dried at temperatures between 80° C. and 200° C., usuallyin air, and ultimately calcined in air for a period of from about onehalf to one and one half hour temperatures from 300° C. to 600° C. Othertechniques, well known to those skilled in the art include sputtering,and other methods of deposition including vapor deposition, electricaldepostion and electrochemical deposition.

The process of this invention is conducted in a slurry usingconventional equipment. The selected catalyst is taken up in the liquidmedium to form a suspension of the catalyst at a concentration of fromabout 10 to 60 percent by volume. Concentrations of from about 25 to 50g/l are preferred.

Suitable temperature range for conducting the process of the presentinvention is from about 200° C. to 350° C., preferably from 225° C. to275° C.

The pressures at which the reaction is carried out can vary over a widerange provided they are selected so that the parameters defined by theabove formulas are observed. Typically, and preferred partial pressuresfor hydrogen, carbon monoxide and steam are as follows:

    ______________________________________                                                      Typical Preferred                                                             psi     psi                                                     ______________________________________                                        Hydrogen        1500 to  0                                                                              750 to 25                                           Carbon monoxide 1500 to 15                                                                              1000 to 100                                         Steam            500 to  1                                                                              300 to 25                                           ______________________________________                                    

Control of the partical pressures of the reactants is used to controlthe number of moles of the reactants, thereby assuring that the abovedefined parameters are observed.

Typically, the reactants are charged into an autoclave containing asuspension of the selected supported catalyst. The autoclave is sealedand brought to the reaction temperature. The pressure increases to thedesired range and then starts to decrease indicating that reaction istaking place.

Those skilled in the art will recognize that reactions of this typeproduce mixtures of products. Thus, for example, in the production ofhydrocarbons the reaction can be conducted within the parameters abovedefined to produce principally ethane, but the reaction mixture willcontain small amounts of other hydrocarbons such as methane, propane andbutane.

The following non-limiting examples are given by way of illustration,

EXAMPLE 1

Platinum is deposited on 10 g zinc oxide by impregnation. This catalystis slurried in a solvent system of 100 ml consisting of 1 parttetrahydrofuran and 5 parts cyclohexane. This is introduced into a 500ml stirred pressure reactor with 12 ml of water. The reactor ispressurized with 500 psi carbon monoxide and 500 psi hydrogen, heated to240° C. and stirred at 750 rpm. After 5 hours, pressure is released andproducts analyzed by gas chromatography. Methanol is the sole product.

EXAMPLE 2

Cobalt is substituted for platinum in Example 1. Hydrocarbon productsincluding methane, ethane, propane and higher products are found.

EXAMPLE 3

Cobalt and copper are deposited from basic solutions onto zinc oxidesupport for use as the catalyst. The products from a process similar tothat of Example 1, include mixed alcohols and hydrocarbons.

Although the present invention is described in terms of specificmaterials and process steps, it will be clear to one skilled in the artthat various modifications may be made consistent with the scope of theaccompanying claims.

The embodiments of this invention in which an exclusive property ofprivilege is claimed are defined as follows:
 1. A method for thereaction of a mixture of carbon monoxide, hydrogen and water for theproduction of at least one carbonaceous product selected from the groupconsisting of hydrocarbons, alcohols and mixture thereof comprisingcontacting the reactants together in a slurry containing a catalystsuspended in a reaction inert liquid medium at a temperature of fromabout 200° to 350° C., the catalyst being on a zinc oxide support andbeing selected from the group consisting of cobalt, nickel, ruthenium,palladium, platinum, iridium, rhodium, copper and mixtures thereof. 2.The method of claim 1, for the production of hydrocarbons wherein theinert liquid medium with catalyst is at a temperature of 225° C. to 275°C., the catalyst on a zinc oxide support is selected from the groupconsisting of cobalt, nickel, ruthenium and mixtures thereof.
 3. Themethod of claim 2 for the production of saturated hydrocarbons whereinthe ratio of reactants being such that:

    (2n+1)≧x≧O

where n being the number of carbon atoms in the product and x being thenumber of moles of water in the reaction mixture.
 4. The method of claim2 for the production of olefinic hydrocarbons wherein the ratio ofreactants is such that:

    2n≧x≧O

where n is being the number of carbon atoms in the product and x beingthe number of moles of water in the reaction mixture.
 5. The method ofclaim 1 for the production of alkanols wherein the inert liquid mediumwith catalyst is at a temperature of 225° C. to 275° C., the catalyst onzinc oxide support being selected from the group consisting ofpalladium, platinum, iridium, rhodium, copper and mixtures thereof andwherein the ratio of reactants is such that:

    2n+1≧x≧O

where n being the number of carbon atoms in the product and x being thenumber of moles of water in the reaction mixture.